1. Technical Field
This invention relates to a medical device for implantation within the human or animal body for the repair of aortic aneurysms.
2. Background Information
In general, as a thoracic aortic aneurysm dilates, its length also increases. Distally this creates a transverse segment above the diaphragm. Proximally, this pushes the distal arch in a cranial direction, often causing the aorta to buckle at the top of the aneurysm. This makes the arch more difficult to traverse with a delivery system. Nevertheless, some solutions have been presented. For example, trackable, kink-resistant sheaths have been developed that will follow a stiff guidewire around almost any bend in the aorta.
However, the more difficult problem is stent-graft implantation. To function effectively, a proximal end of the stent graft has to occupy a co-axial position with the distal aortic arch of the implantation site while the rest of the graft has to traverse the long axis of-the aneurysm, which lies in a very different direction. The bending moment imposed upon the proximal end of the stent-graft is required to overcome the stiffness of the rest of the implant and induce bending but not kinking. Naturally, kinking may restrict the flow of blood through the implant. In general, the longer the implantation site, the greater the influence of aortic axis on proximal stent axis. The stiffer the stent graft, the greater the resistance to that influence.
Flexibility of a stent graft can be limited by a number of factors. Flexibility is sometimes sacrificed to provide dimensional stability or column strength. Some stents lack flexibility when there is little or no differential shortening or lengthening. Attaching a graft to the stent can also limit the flexibility.
Flexible stent-grafts typically have short, widely spaced stents. Most designs do not permit stent overlapping because the ends are all attached to fabric to prevent movement and graft erosion.
The length of the implantation site may also be limited by the proximity of the aneurysm to the arch vessels because covered stents cannot encroach on arterial orifices without causing occlusion, which can lead to a stroke. Uncovered stents have been known to cause erosion of the soft, curved, mobile aortic arch.